Interpretive Summary: Fire ants are stinging invasive ants from South America that infest over 129.5 million hectares in the southern United States. Scientists from USDA-ARS Center for Medical, Agricultural, and Veterinary Entomology have summarized research on fire ant pathogens to illustrate the effort to understand their biology and facilitate their utilization as biological control agents against fire ants. Microscope examinations of fire ants in South America during the 1970s and 1980s led to discoveries of fire ant infecting fungi and microsporidia (protozoa), of which three have been studied extensively for fire ant control. A fungus, Beauveria bassiana, caused fire ant mortality, but infections did not spread to queens and transmission between colonies was not evident. One of the microsporidia, Thelohaina solenopsae, has been found in the U.S. and has been shown to spread naturally and debilitate colonies. Colony decline also has been associated with another microsporidium, Vairimorpha invictae, and is currently being evaluated for possible release in the U.S. Through the use of molecular techniques, viruses have been discovered and characterized from fire ants collected in the U.S. Solenopsis invicta virus 1 can be transmitted easily to uninfected colonies and colony death often results. The virus apparently causes persistent, asymptomatic infections that actively replicate when the host is stressed. The biological control of fire ants, and especially by pathogens, is viewed by some as the only sustainable tactic for suppression of the ubiquitous fire ants.

Technical Abstract:
Fire ants, Solenopsis richteri and Solenopsis invicta, are stinging invasive ants from South America that infest over 129.5 million hectares in the southern United States. In the southern U.S., eradication is no longer considered possible and toxicant-based fire ant baiting is currently the primary method of control. The use of parasites and pathogens as biological control agents in combination with insecticide baits is now encouraged. The biological control of fire ants, and especially by pathogens, is viewed by many as the only sustainable tactic for suppression of the ubiquitous fire ants. Microscopic-based surveys conducted in South America during the 1970s and 1980s led to the discovery of fire ant infecting fungal isolates and microsporidia. Three of these microorganisms have been studied extensively: Beauveria bassiana 447, Thelohania solenopsae, and Vairimorpha invictae. B. bassiana 447 caused fire ant mortality, but infections did not spread to queens and intercolony transmission was not evident. T. solenopsae has been found in the U.S. and has been shown to spread naturally and debilitate colonies. Colony decline also has been associated with V. invictae, and is currently being evaluated for host specificity and possible release in the U.S. Through the use of molecular techniques, fire ant infecting viruses have been discovered and characterized from fire ants collected in the U.S. Solenopsis invicta virus 1 can be transmitted easily to uninfected colonies and colony death often results. The virus apparently causes persistent, asymptomatic infections that actively replicate when the host is stressed. Research on fire ant-specific microsporidia and viruses, as well as other fire ant entomopathogens are summarized to illustrate the effort to understand their biology and facilitate their utilization as biological controls for fire ants.